DE69415724T2 - Method and device for monitoring lung function - Google Patents

Abstract

A method of measuring lung function in a subject includes connecting a pneumotach in series with the subject's airway and taking pressure readings upstream and downstream of a fixed resistance element (30) in the pneumotach. Second pressure readings are taken with a second resistance element (46) in place, and the sets of readings are processed to calculate characteristic flow curves for the subject's airway. First and second time constants ( tau ) are calculated from the flow curves, and are used to calculate values of lung compliance (Clt) and airway resistance (Raw). The invention extends to a pneumotach device with a fixed first resistance element and a removable second resistance element. The invention also extends to apparatus for monitoring lung function including the pneumotach device, together with a pair of pressure sensors (50) and a processor circuit (52, 54, 56, 58, 60) for carrying out the necessary calculations of the time constants, and the lung compliance and airway resistance values. A display (62) is provided for giving an indication of the calculated values. <IMAGE>

Description

BACKGROUND OF THE INVENTION

This invention relates to a method for measuring lung function in a living being and a device for use in carrying out the measurement.

Measuring lung function in humans and other animals has a number of uses. In patients with emphysema or other respiratory diseases, monitoring lung function can be useful both in treating the disease and in conducting an investigation. It can also be used to monitor lung capacity and function in athletes. There are a number of other uses for such measurements.

Two important parameters in measuring lung function are airway resistance (Raw) and lung and thorax distensibility (CIt). These parameters are analogous to electrical resistance and capacitive reactance and define a time constant r = Raw · CIt. Until now, airway resistance has always been measured by using a body plethysmograph, while lung and thorax distensibility has been measured by using a swallowed balloon. This is undoubtedly a cumbersome and uncomfortable procedure.

US-A-5 060 655 describes a pneumotach for measuring respiratory flow. The pneumotach has an air passage with a throttling sieve arranged in it. The flow rate of the air through the passage is calculated from an effective pressure across the sieve.

It is an object of the invention to provide an alternative method and an alternative device for measuring lung function.

SUMMARY OF THE INVENTION

According to the invention, a method for monitoring lung function in a living being comprises the following steps:

the series connection of a pneumotach with the airway of the living being,

taking a plurality of initial airway pressure readings upstream and downstream of a first resistance element in the Pneumotach,

inserting or removing a second resistance element into the Pneumotach so that the total resistance of the airway of the living being is changed,

taking a plurality of secondary airway pressure readings upstream and downstream of the first resistance element in the Pneumotach,

processing the plurality of first and second pressure readings to calculate the first and second characteristic flow curves for the airway of the subject, respectively,

deriving the first and second time constants of the first and second flow curves, and

calculating values of lung distensibility and airway resistance for the living being from the first and second time constants.

A plurality of first and second airway pressure readings are used to calculate the first and second airway flow curves, respectively.

The airway flow curves are preferably curves of airflow over time from which the lung time constants can be derived According to the invention, the pneumotach device for monitoring lung function in a living being further comprises:

a body defining a conduit therein for connection in series with the airway of the living being,

a first resistance element in the line, and

first and second openings in the body communicating with the line on either side of the first resistive element;

and a second resistance element movable between an operative position in the conduit in which it increases the resistance to the flow of gas in the conduit and a rest position in which it does not substantially affect the flow of gas in the conduit, wherein the second resistance element is removably housed in a slot in the body intersecting the conduit.

The first and second resistance elements may be perforated plates having a predetermined actual opening size.

The second resistance element is preferably designed such that it can be removed from the body in its rest position.

The openings in the body can be designed to connect to respective pressure sensors via pipes and hoses.

Alternatively, the body may be configured to accommodate pressure sensors in or adjacent to the openings.

In both cases, the pressure sensors are arranged to measure the pressure differential across the first resistive element, and from this measurement the air flow in the line can be calculated.

The invention extends to a device for monitoring lung function in a living being, which comprises the pneumotach device as defined above and further comprises:

first and second pressure sensors in communication with the first and second openings in the body and arranged to produce respective output signals corresponding to respective pressure readings, and

Processing means for receiving first and second sets of output signals from the sensors corresponding to the pressure readings with the second resistance means in the resting and operating positions, for calculating first and second characteristic flow curves for the airway of the subject, for deriving first and second time constants from the first and second flow curves, and for calculating values of lung distension and airway resistance therefrom.

The device may further comprise indicator means, such as a digital or graphic display, to indicate one or more of the measured or calculated values.

The processing means may comprise respective amplifiers for amplifying the outputs of the first and second pressure sensors, at least one analog-to-digital converter for converting the amplified output signals into a digital form, and a microprocessor for performing the necessary calculations and generating output and display signals as required.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which

Figure 1 is an exploded side sectional view of a pneumotach device according to the invention;

Fig. 2 is a partial sectional side view of the device of Fig. 1 in an assembled condition;

Fig. 3 is a simplified schematic block diagram of the device for measuring lung function incorporating the pneumotach of Figs. 1 and 2;

Fig. 4 is a simplified graphical representation illustrating the calculation of the relevant parameters by the device of Fig. 3;

Fig. 5 is a schematic diagram illustrating the airway parameters relevant to forced expiration;

Figure 6 is a simplified flow diagram illustrating the operation of the processing means of the apparatus of Figure 3;

Fig. 7 is a schematic diagram of a preamplifier circuit of the device ;

Fig. 8 is a schematic diagram of the microprocessor and the analog-digital circuit of the device; and

Fig. 9 is a schematic diagram of a flip-flop and a timing circuit of the device.

DESCRIPTION OF AN EMBODIMENT

The device illustrated in Figures 1 and 2 of the drawings is a modified pneumotach which can be inserted in series with the airway of a subject and which allows two different predetermined resistance elements to be placed in series with the subject's airway to enable accurate measurements to be made from which lung function information can be calculated.

The pneumotach has a body 10 which includes an inlet section 12, a central section 14 and an outlet section 16 which are generally circular in plan and each having a circular central opening so that the body as a whole defines a central tubular conduit 18. The inlet section 12 and outlet section 16 of the body 10 have respective tubular ends 20 and 22 for connecting the tubes or hoses to the pneumotach. In Fig. 2, a flexible hose 24 is connected to the inlet section 12 and is shown (in Fig. 3) inserted into the mouth of a human subject 26. Typically, the pneumotach 10 forms part of an anesthetic circuit or a more complex measuring arrangement than that illustrated schematically in Fig. 3.

Referring again to Figures 1 and 2, the inlet section 12 of the body 10 forms a circular seat 28 concentric with its bore which receives a resistance element in the form of a perforated plate 30. The plate 30 typically comprises a metal disk having perforations of a predetermined number and size therein so as to provide a predetermined degree of resistance to air or gas flow through the conduit 18 of the pneumotach. The resistance element 30 is clamped in place in the seat 28 when the inlet section and the central section 14 of the pneumotach are secured together as shown in Figure 2.

Openings 32 and 34 are formed in the inlet portion 12 and central portion 14 of the body, respectively, and terminate in pins 36 and 38 having enlarged heads for retaining hoses 40 and 42. Openings 32 and 34 extend radially through the body portions 12 and 14 and communicate with the central conduit 18.

A radially extending slot 44 is formed in the outlet section 16 of the pneumotach body, which intersects the line 18. This slot accommodates a second resistance element 46, which is similar to the resistance element 30, but has a tip 48 or other means for gripping an edge thereof, thereby enabling a user of the device to grasp it and insert it into or remove it from the slot 44. In Figure 2, the second resistance element 46 is shown in place in the slot 44 so that the first and second resistance elements are effectively arranged in series with the subject's airway. When the second resistance element 46 is removed from the slot 44, only the first resistance element 30 remains in the patient's airway so that it can be seen that the total resistance introduced into the subject's airway can be varied between a value R₁ corresponding only to the first resistance element 30 and a second value R₁ + R₂ corresponding to both resistance elements 30 and 46.

In a prototype of the invention, the resistance element 30 had a resistance value of about 0.35 cm H₂O per liter per second, while the value of the resistance element 46 was about 1.0 cm H₂O per liter per second.

Referring now to Fig. 3, the tubes 40 and 42 connected to the pneumotach of Figs. 1 and 2 are shown connected to a differential pressure (DP) transducer 50. The transducer provides electrical output signals proportional to the pressure differential across the first resistive element 30 in the line 18 of the pneumotach. The output of the transducer 50 is amplified by a preamplifier 52 and the amplified output signal is provided to an analog-to-digital (A/D) converter 54. The A/D converter digitizes the amplified output signal from the transducer and the digital output signal is provided to a microprocessor 56 with associated read-only memory (ROM) 58 and random access memory (RAM) 60. A digital display 62 (or other display such as a graphic display) is designed to be driven by an output signal from the microprocessor 56 or an associated display driver.

The electronic circuit of Fig. 3 is illustrated in more detail in Figs. 7, 8 and 9, which are schematic diagrams of the circuit arrangements used in the prototype of the invention.

In Fig. 7, the differential pressure transmitter 50 is a device of type LT1014 connected to respective pairs of LM 324 Op Amps U2A, U2B, U2C and U2D, which correspond to the preamplifier 52 in Fig. 3. The respective amplified output signals from the preamplifiers are applied to pins 1 and 2 of a connector 64 which is also used to conduct current from a power supply circuit to the pressure sensor and amplifiers. The circuit of Fig. 9 includes a threshold detector USA which receives the output of the preamplifier circuit of Fig. 7 through a differentiating circuit comprising a resistor R5 and a capacitor C7. The output of the differentiator circuit is a rapidly rising initial flow signal which is shaped and inverted by the threshold detector U5A and applied to the trigger input of a timer circuit U8 of type 555. This starts the timer circuit which establishes a 5 second delay chosen to correspond to the maximum probable expiration time to be measured.

The output of the timing circuit is a positive square pulse applied to the base of a transistor Q2. The transistor Q2 is turned on, turning off a second transistor Q1, and a 5 second trigger signal is generated.

The trigger signal is used by the circuit of Fig. 8 to start another oscillator circuit carried around a type 555 timer U7 and designed to run at 200 Hz. The trigger signal also starts a microprocessor U1 which turns on an A/D converter U4 and stores the digital data output from the A/D converter in RAM memory.

Fig. 4 is a graph showing two characteristic flow curves for the airway of a subject. As can be seen, the curves have a generally logarithmic decrease from an initial volume V₀ to a smaller volume that varies with the time in which the subject exhales. In the uppermost curve, the velocity of airflow in the subject's airway decreases due to the presence of the second resistance element R₂, while while the lowest curve shows a faster exhalation flow due to the absence of the second resistance element R₂.

When the pneumotach device is connected in series with the subject's airway, a flow of air through the pneumotach due to exhalation results in a pressure difference across the first resistance element 30, and the output signal of the pressure transducer 50 corresponding to this pressure difference indicates the flow of air through the pneumatics.

The microprocessor 56 of Fig. 3 operates under the control of software stored in RAM 58 according to the simplified flow chart of Fig. 6. The device has two basic modes of operation, namely a "passive exhalation" mode and a "forced exhalation" mode. First, dealing with the passive exhalation mode, the pressure readings from the pressure transducers 44 and 46 are continuously converted to digital values and fed to the microprocessor which stores the values from each transducer in a data array for a period of several seconds. This effectively stores the upper and lower curves illustrated in Fig. 4. In each case, a start time t1 is determined which corresponds to 0.9 x V0. A second time t2 is calculated so that the difference t2 - t1 = r. The lung time constant r is the time (in seconds) required for the value of V in the graph of Figure 4 to fall to 1/e or 36.8% of its initial value.

The relationship can be expressed as follows:

From the known relationship:

τ = RawCIt

and from the two sets of readings taken with and without the second resistance element in place in the Pneumotach, the Airway resistance Raw and the distensibility of the lungs and thorax CIt are calculated.

The device of Fig. 3 can also be used in a forced exhalation mode. The schematic diagram of Fig. 5 provides an indication of the airway parameters relevant to the forced exhalation mode. In Fig. 5, the circular section 64 on the left of the diagram represents the alveolar sacs of the lung, which join together to form the elastic element or stretch of the lung, which in turn is responsible for increasing recoil of the alveoli as the lung volume increases. The narrow tubular section 66 to the right of the circular section 64 represents the conducting airway, while the rectangular box 68 represents the thorax. The opening 70 at the right end of the box 68 represents the opening of the mouth.

Pressure inside the chest is transmitted to the outside of the alveoli. During exhalation, the pressure in the alveoli is highest because the recoil pressure of the alveoli is added to the intrathoracic pressure. If the pressure at the mouth is nominally zero, a pressure gradient results when airflow begins.

At one point along the conducting airway 66, the pressure inside and outside the airway will be equal. This is the equal pressure point (EPP) indicated in the diagram. The airflow upstream of this point is never compressed and is called the upstream segment Ru, while the downstream segment is called Rd. The relative lengths of the upstream and downstream segments vary depending on the intrathoracic pressure, but they have a fixed relationship when the airflow becomes effort independent. During forced expiration, there is an intrathoracic pressure increase and a lung volume decrease as flow begins. As flow decreases, the flow curve is divided into an effort dependent first part and an effort independent second part where the flow rate cannot be increased by increased intrathoracic pressure. Experimental studies have shown that the middle third of the flow curve has an essentially linear relationship with the exhaled volume which corresponds to the case when the product of strain and resistance is a constant.

The device software can integrate the recorded flow readings with volume and plot the flow versus volume. If the flow is not turbulent, this relationship is a straight line, with the direction coefficient being the time constant. The first and last parts of the flow/volume curve are often turbulent and the central straight section of the curve can be selected to simplify the necessary calculations.

This can be done visually using a graphical display of the flow/time curve or automatically.

If increasing intrathoracic pressure does not cause an increase in flow, then the rate of rise of Rd must be equal to the rate of rise of intrathoracic pressure. In this case, control of airflow is determined by the lung distension CI and Ru. The product RuCI is the lung's time constant for forced expiration. This is a useful index of lung function because it is linearly related to the severity of obstructive lung diseases such as asthma and emphysema and limiting diseases such as sarcoidosis.

In the passive expiratory mode, the invention operates by recording time constant curves (preferably pull-breath curves or expiratory curves under anesthesia) with two different known resistance values in the expiratory circuit. In a "monitor" mode, used during anesthesia, a reading of CIt · Raw is provided which is updated every minute. In a "measure" mode, the Raw and CIt measurements are displayed separately. The software incorporated in the device checks the measurements until the discrepancy between successive measurements is less than 10%. The measured results are then displayed.

In the forced exhalation mode, CI · Ru is measured. This reading is particularly useful in the field and, for example, in a polyclinic where quick results are needed for numerous patients.

The method and device described allow relatively rapid and accurate measurements of the airway resistance and the lung and chest expansion of a living being to be made without having to perform laborious and unpleasant procedures. The device can be conveniently made small and portable for use in the field or in operating rooms.

Claims (14)

1. A method for monitoring lung function in a living being, comprising the following steps: connecting a pneumotach (10) in series with the airway of the living being (26); taking a plurality of first airway pressure readings upstream and downstream of a first resistance element (30) in the pneumotach; inserting or removing a second resistance element (46) into the pneumotach so that the overall resistance of the airway of the subject is changed ; taking a plurality of second airway pressure readings upstream and downstream of the first resistance element (30) in the pneumotach; processing the plurality of first and second pressure readings to calculate first and second characteristic flow curves for the subject's airway, respectively; deriving the first and second time constants τ of the first and second flow curves; and calculating values of the lung distensibility CIt and the airway resistance Raw for the living being from the first and second time constants. 2. A method according to claim 1, wherein the airway flow curves are curves of air flow versus time from which the time constants τ can be derived. 3. A method according to claim 2, wherein the time constants are determined from the relationship where t₁ and t₂ are the start and end times for the airway flow curves and V₁ and V₂ are the corresponding lung volumes, respectively. 4. A method according to claim 2 or 3, wherein the lung and chest distensibility CIt and the airway resistance Raw are calculated from the relationship τ = RawCIt. 5. A method according to claim 2, wherein the time constants are derived from the slope of a flow/volume expiratory curve. 6. Pneumotach device for monitoring lung function in a living being, comprising: a body (10) defining a conduit (18) therein for connection in series with the airway of the living being (26); a first resistance element (30) in the line; and first and second openings (32, 34) in the body communicating on both sides with the line of the first resistive element; characterized by a second resistance element (46) movable between an operative position in the conduit in which it increases the resistance to the gas flow in the conduit and a rest position in which it substantially does not affect the gas flow in the conduit, wherein the second resistance element (46) is removably housed in a slot (44) in the body (10) intersecting the conduit (18). 7. Pneumotach device according to claim 6, wherein the first and second resistance elements (30, 46) are perforated plates having a predetermined actual opening size. 8. Pneumotach device according to claim 6 or 7, wherein the second resistance element (46) is designed so that it is removable from the body (10) in its rest position. 9. Pneumotach device according to one of claims 6 to 8, wherein the openings (32, 34) in the body (10) are designed for connection to respective pressure sensors (50) via pipes or hoses (40, 42). 10. Pneumotach device according to one of claims 6 to 8, wherein the body (10) is adapted to receive pressure sensors in or adjacent to the openings (32, 34). 11. A device for monitoring lung function in a living being, which comprises the pneumotach device according to any one of claims 6 to 10 and further comprises: first and second pressure sensors (50) in communication with the first and second openings (32, 34) in the body (10) and arranged to produce respective output signals corresponding to respective pressure readings; and Processing means (52, 54, 56, 58, 60) for receiving first and second sets of output signals from the sensors corresponding to the pressure readings with the second resistance means (46) in the resting and operating positions; for calculating first and second characteristic flow curves for the airway of the subject; for deriving from the first and second flow curves first and second time constants τ; and for calculating values of lung distension CIt and airway resistance Raw therefrom. 12. Pneumotach device according to claim 11, wherein the pressure sensors (50) are arranged so that the pressure differential across the first resistance element (30) can be measured, from which measurement the air flow in the line (18) can be calculated. 13. A device for monitoring lung function according to claim 11 or 12, further comprising indicator means (62) for displaying one or more of the measured or calculated values. 14. A lung function monitoring device according to any one of claims 11 to 13, wherein the processing means (52, 54, 56, 58, 60) comprises respective amplifiers (52) for amplifying the outputs of the first and second pressure sensors (50), at least one analog-to-digital converter (54) for converting the amplified output signals into a digital form, and a microprocessor (56) for performing the necessary calculations and generating output or display signals as required.